1. Field of the Invention
This invention generally relates to the field of joining two substrates with a sealant, and more particularly the art of using laser radiation for curing the sealant during the manufacturing of LCD (Liquid Crystal Display) panels.
2. Description of the Related Art
LCD displays are manufactured by joining two substrates to one another with liquid crystal material between the two substrates. Both substrates are typically glass. One method for joining the two substrates to form a panel is related to U.S. Pat. No. 5,263,888 by Ishihara et al in which an ODF (One Drop Fill) method for assembling liquid crystal flat panel displays is described. Using this method, the liquid crystal is deposited on one of the substrate's interior to the glue seal. The glue seal is pre-deposited near the peripheral edge of the substrates. The two substrates are brought in contact with one another. The glue seal must be cured rapidly in order to seal the entire periphery. Accordingly, a need exists for a method of curing the glue seal for use in a high quality, high volume but low-cost manufacturing environment.
The strength of the cured sealant holding the two substrates together is also prime consideration. Since the liquid crystal material is inside of the panel prior to cross linking, it is not possible to utilize the standard thermal baking of the glue seal for curing since the temperature required for thermal cross linking generally exceeds the temperature the liquid crystal can tolerate. Accordingly, a need exists for a non-thermal method for curing the sealant material allowing the sealant method to stay below a temperature that is harmful to the liquid crystal material.
Furthermore, the thermal process for curing the sealant generally takes on the order of 1 hour or more. During this time, there is a great chance for intermixing of the non-epoxy glue seal and the liquid crystal, which leads to eventual poisoning of the liquid crystal resulting in severe defects in the operation of the display panel. Accordingly, a need exists for a fast method of curing the sealant, which does not allow sufficient time for intermixing of the glue sealant in a liquid state and the liquid crystal material.
Another method for curing non-epoxy sealant is to incorporate photo initiators in the sealant rather than thermal initiators. Lasers have been shown to offer both speed and efficiency when photo initiators are present in the non-epoxy glue sealant. The use of a laser allows for this efficient light source to rapidly cure the sealant without an unwanted temperature rise extending into the liquid crystal. There are many factors when using lasers that must be considered including the laser frequency, whether the laser is pulsed versus non-pulsed, the power of the laser, and the exposure time and direction of the laser. An attempt must be made here to find several commercially available solutions that will work and yet provide a cost effective manufacturing solution. Accordingly, a need exists for the determination of a viable selection of the type of laser and the exact exposure method.
One disadvantage of non-thermal curing methods is that the light source must have a clear light path to the sealant. LCD panels are constructed with circuit traces that exit the edge of the panel. These circuit traces are used by the display driver to effect the intended image on the panel. However the traces cause blocking or shadowing of the sealant that is under the traces. Accordingly the need exists for a method to allow for the light source to be able to reach the sealant that is under the circuit traces.